In a balloon dilatation procedure, a catheter carrying a balloon on its distal end is placed within a body cavity of a patient and is inflated to dilate the cavity. The procedure is commonly employed to dilate a stenosed artery, and in particular to dilate obstructed coronary arteries. Dilation procedures also are performed in peripheral blood vessels, the heart valves, and in other portions of the body.
There are several desirable features for a dilatation balloon. The balloon should have a maximum and controllable inflated diameter. Typically, a physician selects a balloon having an inflated diameter which corresponds to the inner diameter of the unobstructed blood vessel adjacent the stenosis to be treated--any expansion beyond this diameter may cause rupture of the vessel. The balloon should have a thin wall so that it can fold down closely about the catheter shaft to a low profile, thereby enabling the deflated balloon to be inserted into and removed from narrow stenoses and passageways. The balloon also needs to be flexible, as stiffness detracts from the ability of the balloon to bend as it is advanced through tortuous passageways, a characteristic sometimes referred to as "trackability." Low stiffness (high flexibility) also enables the balloon to be folded easily within the patient's body when the balloon is deflated. In this regard, it should be understood that when the balloon is deflated, it typically tends to collapse into a pair of wings which, if not sufficiently flexible, will not fold or wrap easily about the catheter body as the deflated balloon catheter is advanced or withdrawn against body tissue. The balloon should also have a sufficiently high burst strength to enable it to impart sufficient dilatation force to the vessel to be treated. However, the burst strength required for different procedures varies considerably because the dilating force of the balloon increases as a function of the diameter of the balloon, without requirinq a corresponding increase in the inflation pressure. Thus, the larger the diameter of the balloon, the lower its burst strength may be while still developing sufficient dilatation force. For example, a 20 millimeter (mm) diameter balloon used in a valvuloplasty procedure need only have a burst strength of about 3 to 6 atmospheres (atm), whereas a 3 mm diameter balloon used in the dilatation of small coronary arteries may require a burst pressure of 10 to 20 atm.
Dilatation balloons have been made from a variety of thermoplastic polymer materials, including polyesters, polyurethanes, polyvinyl chloride, thermoplastic rubbers, silicone polycarbonate copolymers, ethylene-vinyl acetate copolymers, ethylene butylene styrene block copolymers, polystyrene, acrylonitrile copolymers, polyethylene, polypropylene, and polytetrafluoro ethylene (PTFE). Each of these materials has different intrinsic properties and may require different processing techniques.
U.S. Pat. No. 4,490,421 to Levy (now Reissue 32,983) describes the processing of a semi-crystalline polyester homopolymer, namely, polyethylene terephthalate (PET), to produce a balloon having superior toughness, flexibility and tensile strength. The balloon is formed by heating a tubular parison in an external mold to a temperature above the orientation temperature, axially drawing and circumferentially expanding the parison to form a balloon and then cooling below the orientation temperature. To heat the parison above the orientation temperature, an external balloon mold is inserted into a heated liquid medium, or a heated liquid is passed through chambers in the mold, such that heat is applied to the exterior surface of the parison and time is allowed for the temperature across the parison sidewall to equilibrate. A relatively thin wall and high strength balloon is produced.
One of the problems with the known heating and expansion techniques for making PET balloons is that it produces a balloon having an optimum (high) tensile strength on the inner surface, but a much lower degree of strength on the outer surface. This varying degree of tensile strength across the sidewall results in a lower overall or "average" tensile strength. Ideally, it would be desirable to achieve the optimum (highest) tensile strength at both surfaces of the balloon and across the wall in order to achieve the highest average tensile strength.
The amount of orientation (and resulting strength) achieved at any point across the sidewall of a balloon made from a semi-crystalline orientable polymer (such as PET), is a function of temperature (higher temperature equals less orientation) and degree of stretch (higher stretch equals higher orientation). Thus, even if the inner and outer diameters of the parison start at the same temperature as intended with the prior art method, the orientation achieved at the inner diameter is greater due to the greater inherent degree of stretch at the inner surface. More specifically, due to the relative differences in thicknesses between the inner and outer diameters of the parison and balloon, the inner surface stretches more, and in most cases quite significantly more, and the degree of stretch is progressively less moving outwardly across the sidewall to the outer surface. Thus, while the inner surface may achieve the optimum (highest) tensile strength possible, the outer surface achieves a much lower degree of stretch and this reduces the overall o average tensile strength. Still further, if temperature equilibrium across the wall is not achieved and the outer surface of the balloon remains at a higher temperature, then the inner surface is oriented to an even greater degree compared to the outer surface and the average tensile strength is even lower.
It is an object of this invention to increase the orientation at the outer surface and across the wall in order to provide a balloon having a higher average tensile strength.
It has been suggested in the art of making non-crystalline carbonated beverage bottles to provide a temperature gradient across the sidewall of the parison in order to prevent stress whitening (i.e., lack of clarity) and low impact strength which occur when the inner surface is stretched more than the outer surface. However, the bottle making method is not suitable for making a much thinner dilatation balloon and it was not known whether a temperature qradient could even be achieved in a very thin wall parison as used to make a dilatation balloon.